Sustained uterine contractions: a cause of hypointense myometrial bulging

Fifteen pregnant women were studied with magnetic resonance (MR) imaging. In 14 women, the myometrium exhibited distinct hypointense bulging on T2-weighted images that distorted the gestational sac and decidua but not the outer uterine coutour. In 12 women, an obvious discrepancy was observed between the inner myometrial configuration on T1-weighted images and that on intermediate and T2-weighted images. In three women who underwent serial T2-weighted studies separated by about 30 minutes during one MR examination, hypointense myometrial bulging was observed at a different site on each study. In two women who underwent follow-up MR study within a week after dilation and curettage, the previously distinct hypointense bulging completely disappeared. These findings confirm the transient and mobile nature of this myometrial bulging. This phenomenon can be attributed to sustained uterine contractions, which partially force the blood out of the uterus. The decrease in blood volume results in a decrease in water content, which accounts for the decrease in myometrial signal intensity on T2-weighted images.